The magnetometers, each of which uses the SQUID comprising a superconducting loop having one or more Josephson junctions, have been proposed(for example, see IEEE. Trans. Devices, ED27, No.10(1980) pp.1896.about.1908.). The magnetometers can detect very weak magnetic flux and can be used in the fields of detection of biological magnetic field, geophysics, surveying of natural resouces and so on.